So far, the prior catalytic cracking arts still use the early riser reactor and reaction-regeneration system, wherein a riser reactor and a regenerator make-up a catalyst recycle system. The riser reactors are mostly 30-36 meters high and some of them are even longer than 40 m. The production process is that in the riser reaction-regeneration system, preheated feedstock enters a riser reactor through the feed nozzle and comes into contact with the high-temperature catalyst coming from a regenerator, vaporizes, and reacts. The catalyst-carrying oil-vapor flows along the riser upwards at an average linear velocity of about 10 mls it reacts while it flows and the reaction takes about 3 seconds. During the reaction procedure, coke generates and deposits on the surface and the active center of the catalyst so that the activity and selectivity of the catalyst drop rapidly. For this reason, the coked catalyst must separate from the oil-vapor in time and enter a regenerator for regeneration and recycle application, thus forming a circuit of the catalyst. The oil-vapor enters a distillation system to separate into products (generally including three products, i.e. catalytic diesel oil, gasoline, and liquefied petroleum gas). Part of the feed oil, which does not convert into light products after once reaction (generally called recycle oil), enters the riser reactor again to carry out reaction. This is the basic process of the catalytic cracking reaction-regeneration system.
Due to the especial characters of heavy oil, varieties of difficulties are brought into the catalytic cracking process. In recent years, the development of the catalytic cracking technology has been focused mainly on the residue fluid catalytic cracking (RFCC) technology. In the prior art, revamps have been made mostly on local parts before or after the riser reactor to achieve certain positive effects. The following are some examples of the main new technologies and their functions:
An atomization technology of heavy feed (nozzle), which improves the contact state between a feedstock and a catalyst to enhance the yield of light oil.
A gas-solid rapid separation technology at the end of the riser to separate the gas-solid quickly and thus reduce the over cracking reaction.
A riser reaction termination agent technology, which shortens the reaction time, reduce the harmful secondary reactions and enhances the yield of light oil.
A high-efficiency multi-stage stripping technology of the spent catalyst, which enhances stripping effects, reduces the yield of coke and increases the yield of light oil.
A two-stage high-efficiency regeneration technology, which enhances the burning of coke, reduces the coke content of the regenerated catalyst and maintains high activity of the catalyst.
A multi-position feeding technology, which treats feedstocks with different characters in different ways and optimizes the reaction process.
A new millisecond catalytic cracking technology, which shortens the reaction time and decreases the secondary reaction.
A descending riser technology for improving the mechanism of the oil/catalyst contact reaction, which is now still in an R & D phase.
All the above prior catalytic cracking arts do not change the general structure of the current riser reactor except the last two which involve a change in the form of the riser reactor. However, the current riser catalytic cracking processes have many disadvantages: (1) Too long a riser leads to an overlong residence time of the oil-vapor in the riser (about 3 seconds), but the catalyst maintains its effective activity and selectivity for only a shorter time (about less than 1 second). Therefore, the improvement of the product distribution and the enhancement of the conversion depth are unfavorable due to the occurrence of lots of thermal reactions and detrimental secondary reactions in the second half of the conventional riser, (2) In the conventional catalytic cracking, the fresh feedstock and recycle oil (recycle oil and oil slurry) react in a same riser. This is very detrimental because the vaporizing character of the two oils are different and their ability to adsorb and react on the catalyst is opposite. The result of competition is that an full and effective reaction can not be achieved, thereby the enhancement of the yield of the light product and the conversion depth are affected (this has been proved by the industrial test on the two-stage riser catalytic cracking technology developed previously by the inventors of the present invention, (3) The content of olefins in the conventional cracked gasoline fraction is high (especially in the cracking of heavy oil) because the activity of the catalyst is very low when a great amount of gasoline is produced, and therefore the olefins in gasoline can not carry out an effective conversion.
The patent submitted by the applicants of the present invention, U.S.20020108887 A1, “Two-stage riser catalytic cracking process” comprises first introducing the high temperature catalyst coming from the regenerator into the lower part of the first riser to contact a feed oil, which vaporizes and reacts, and separating the partly deactivated half-spent catalyst between the two stages after 1 second for regeneration and recycle, providing the regenerated catalyst to the second riser, which comes into contact with the oil-vapor coming from the intermediate separator, said oil-vapor flowing upwards together with the catalyst and continuing the catalytic cracking reaction. Although this process nay improve the distribution and quality of the product, the catalytic cracking reaction of the three streams, fresh feedstock, recycle oil, and cracked oil restricts each other because they need different reaction conditions. Besides, the catalyst needed for cracking heavy oil to produce light oil is completely different from that needed for further cracking gasoline to produce low olefins. Therefore, this process can not achieve different production aims in the catalytic cracking process to produce light oil, or enhance the selectivity of gasoline cracking to ethylene-propylene, or enhance the yield of propylene.